This invention relates to an electrical tooth cleaning apparatus, and, more particularly, to electrical toothbrushes which oscillate smoothly and wherein the operation of electric motors is not stopped even when an user presses the toothbrush down too hard against the teeth or the gums.
FIGS. 1 and 2 show known electrical toothbrushes. FIG. 1 shows a section of a motion transformer disclosed in Japanese Publication No. 59-37081. In the disclosure, when an electric motor is running, an eccentric pin is set in a rotary motion, so that a fork driven by an eccentric pin and thereby an oscillatory toothbrush holding shaft carrying a toothbrush in a non-rotatable manner perform an oscillating movement about its longitudinal axis.
FIG. 2 shows a section of a motion transformer disclosed in U.S. Pat. No. 4,326,314, which has the same construction as that shown in Japanese Publication No. 59-37081, except that an eccentric pin extends in a freely displaceable manner through the diametrical bore of a ball of a ball joint which forms a guide member which is freely displaceable within the longitudinal slot of a fork.
In U.S. Pat. No. 4,413,199, a water-tight switch for switching an electromotor on and off is provided.
The prior art has many disadvantages. First, the direct contact of the eccentric pin with the fork during operation results in severe wear on the contact surface, thus shortening the lives of the toothbrushes. Secondly, power loss due to wear is very large. Thirdly, the long direction-turning time during oscillation results in a non-regular swing. Fourthly, when the toothbrushes are overloaded, they may be broken or the electric motor may stop. Fifthly, noise is produced during the frictional sliding movement of the eccentric pin along the fork member.